1. Field of the Invention
This invention relates to lubricating oil compositions, and more particularly to lubricating oil compositions which contain the metal salt or amine salt of thiophosphate or of phosphate and are decreased in sulfur content and excellent in the ability to maintain the total base number of lubricating oil composition.
2. Description of the Prior Art
Zinc dialkyldithiophosphate (ZDTP) has excellent anti-wear and anti-oxidation properties and thus has been used as an essential additive in lubricating oils for such as internal combustion engines or hydraulic oils and in every sort of lubricating oils.
On the other hand, sulfur-based additives such as zinc dialkyldithiocarbamate(ZDTC) or others are used in lubricating oil to keep anti-wear property instead of ZDTP_as disclosed in Japanese Patent Laid-Open Publication Nos. 52-704, 62-253691, 63-304095, and 6-41568 and Published Japanese Translation Nos. 62-501572, 62-501917, and 1-500912. The lubricating oils disclosed in these publications contain a large quantity of sulfur similarly to those containing ZDTP. Such lubricating oils are poor in oxidation stability and tend to be acceleratingly decreased in total base number of the composition.
After an extensive research and study on the ability to maintain the base number of lubricating oils containing a large amount of sulfur in the process of the degradation, it was found that the oxidation or thermal decomposition of a compound containing sulfur-based additives, such as ZDTP results in the formation of sulfuric acid which significantly decreases the total base number of the composition and deteriorates the high-temperature detergency at a temperature exceeding 300xc2x0 C. It was also found that when recent low-sulfurized gasolines and gas oils, or alternative fuels such as LPG and natural gas are used as fuel particularly in an internal combustion engine, the decomposition of the sulfur-based additive such as ZDTP itself significantly affects the total base number maintaining properties and high-temperature detergency of the lubricating oil. Therefore, it becomes necessary to optimize the wear inhibitor such as ZDTP so as to obtain a longer drain-interval oil than conventional oils while keeping the anti-wear properties thereof. Furthermore, organic molybdenum compounds such as molybdenum dithiocarbamate and molybdenum dithiophosphate are found to be most effective in order to impart fuel efficiency and thus have been used. However, since these compounds contain a large amount of sulfur, they can not improve the total base number maintaining properties and high-temperature detergency and thus fail to obtain both long drain properties and fuel efficiency.
The object of the present invention is to provide a lubricating oil composition which can maintain or enhance anti-wear properties even though decreased in the amount of conventional ZDTP or containing no ZDTP at all and which has excellent long drain properties by suppressing the decrease of the total base number resulting from the deterioration of the lubricating oil. Another object of the present invention is to provide a lubricating oil composition with high-temperature detergency and fuel efficiency and low sulfur content.
After an extensive research and study made so as to solve the foregoing problems, the present invention was achieved by finding that the use of specific phosphorus-containing compounds represented by formulae (1) and/or (2) described hereinafter can produce a lubricating oil composition which can suppress the decrease of the base number resulting from the deterioration of the lubricating oil while maintaining anti-wear properties which are substantially equivalent to or better than those of ZDTP and are excellent in high-temperature detergency and fuel efficiency.
According to the present invention, there is provided a lubricating oil composition which comprises a lubricating base oil and (A) at least one compound selected from the group consisting of compounds represented by the formula 
wherein R1, R2, R3 and R4 are each independently hydrogen or a hydrocarbon group having 1 to 30 carbon atoms, X1, X2, X3 and X4 are each independently oxygen or sulfur, but at least one of them is oxygen, and Y1 is a metal atom; and compounds represented by the formula 
wherein R11 and R12 are each independently hydrogen or a hydrocarbon group having 1 to 30 carbon atoms, X11 and X12 are each independently oxygen or sulfur, but at least one of them is oxygen, U is a monovalent metal ion, an ammonium ion or a proton, and k1 is an integer of 1 to 20.